Reduction of the thiophene nucleus



United States Patent O" REDUCTION OF THE THIOPHENE NUCLEUS IrvingWender, Pittsburgh, Pa., and Milton Orchin, Cincinnati, Ohio, assignorsto the United States of America as represented by the Secretary of theInterior No Drawing. Filed Apr. 2, 1957, Ser. No. 650,283

Claims. (Cl. 260-3322) (Granted under Title '35, US. Code (1952), see.266) This invention herein described and claimed, may be manufacturedand used by or for the Government of the United States of America, forgovernmental purposes without the payment of any royalties thereon ortherefor.

This invention relates to the hydrogenation of the thiophene nucleus inthe thiophenecarboxylate compounds by means of CO and H while employinga cobalt carbonyl catalyst.

In our previous patent, US. 2,587,671, issued March 4, 1954, we haveshown that thiophene compounds containing aldehyde, ketone and alcoholgroups may be reacted with hydrogen and carbon monoxide in the presenceof a metal carbonyl catalyst, so that the oxygen containing side chainsare completely reduced. We have now found, surprisingly, that thiophenecarboxylates are reduced only in the ring, and that the carboxylate sidechain remains unafiected.

The reaction may be represented as follows:

where R R R and R may be hydrogen or an organic radical, such as aryl,alkyl, aralkyl, cycloaliphatic, or carbon to carbon saturatedheterocyclic radical, and may be the same, or difierent.

The active catalyst for this reaction is either cobalt octacarbonyl[Co(CO) or cobalt hydrocarbonyl Preformed carbonyl may be employedalone, or if desired, a cobalt salt, either organic or inorganic, orfinely divided cobalt metal may be added together with the preformedcarbonyl to the reaction mixture. In the latter case the cobalt carbonylformed in situ under the reaction conditions further assists thereaction. Other carbonyls of metals of the eighth periodic group,particularly iron, may be employed. However, the cobalt compound ispreferred since it is easier to handle and has a higher activity.

Formation of the entire cobalt carbonyl catalyst in situ by the use of acobalt salt, e.g., cobalt acetate, in

3,002,002- Patented Sept. 26, 1961 7 tion, a mixture of hydrogen andcarbon monoxide-must be employed. The carbon monoxide does notparticipate directly in this reaction, but apparently functions toprevent decomposition of the cobalt carbonyl under the reactionconditions, and to assure the formation of the carbonyl where it isformed in situ. At a temperature of 180 C. a partial pressure of carbonmonoxide in excess of 300 p.s.i., is necessary to assure the stabilityof cobalt carbonyl.

With these considerations in mind, ratios of H :CO of from 5:1 to :1 maybe employed. The preferred range however is 1:1 to 4:1.

The reaction temperature may vary between 110 C. and 220 C., andpreferably between 120 C. and 180 C. Within these ranges the optimumtemperature will vary with the particular starting material and theotherreaction conditions. Elevated pressures of at least 500 p.s.i. andpreferably 1500 to 3500 psi. are employed.

The following specific example is given to illustrate the 20 invention:

the reaction mixture does not generally give good re- 6 Reduction ofethyl 24hi0phenecarb0xylate.42.5 g.v

(0.27 mole) of ethyl 2-thiophenecarboxylate, 50 ml. of benzene, 1.3 g.of dicobalt octacarbonyl, and 2g. of cobaltous carbonate were placed inthe autoclave. Synthesis gas (211 2100) was admitted until the pressurewas 3200 psi. and the autoclave was heated at 185 for 3 hours; 0.7 moleof gas were absorbed in this interval.

Distillation of the reaction products yielded 28.8 g. of a fractionboiling at 1l3-117 at 26 mm. Hg. Infrared examination of this fractionshowed that 11.4 g. (27%) of this mixture was unreacted startingmaterial and 17.4 g. (40%) was ethyl Z-thiacyclopentanecarboxylate.

The reduction of ethyl Z-thiophenecarboxylate may be represented asfollows:

From the above description it is apparent that the present inventionprovides a convenient and relatively inexpensive method for reducing thethiophene nucleus in thiophene carboxylate compounds. The catalysts forthe reaction are cheap and easily prepared, while the reducing gas canbe any of the available technical mixtures of carbon monoxide andhydrogen, such as water gas.

It is to be understood that the above description together with thespecific examples and embodiments described, are intended only toillustrate the invention, and that the invention is not to be limitedthereto, nor in any way except by the scope of the appended claims.

We claim:

1. A method for reducing a lower alkyl Z-thiophenecarboxylate to thecorresponding lower alkyl Z-thiacyclopentanecarbox-ylate whichcomprises, reacting said lower alkyl 2-thiophenecarboxylate withhydrogen in the presence of a cobalt carbonyl catalyst at a temperatureof about C. to about 220 C. under a pressure of at least about 500p.-s.i., while maintaining a concentration of carbon monoxide in thereaction zone at least sufficient to prevent decomposition of the cobaltcatalyst.

3 4 2. A method as in claim 1 wherein the reaction zone References Citedin the file of this patent is maintained at a temperature of about 120C. to about 180 C. under a pressure of about 1500 p.s.-i. to aboutUNITED STATES PATENFIS 3500 p.s.i. 2,506,536 Baker May 20, 1950 3. Amethod as in claim 1 wherein a cobalt salt is 5 present in the reactionzone. FOREIGN PATENTS 4. A method as in claim 1 wherein finely dividedco- 405,017 Germany Feb. 14, 1925 halt metal 1s present m the reacttonzone. OTHER REFERENCES 5. The method of claim 1, wherein the lower alkylZ-thiophenecarboxylate is ethyl Z-thiophenecarboxylate. 10 Wender etal.: J.A.C.S., vol. 72, pp. 4375-78 (1950).

1. A METHOD FOR REDUCING A LOWER ALKYL 2-THIOPHENECARBOXYLATE TO THECORRESPONDING LOWER ALKYL 2-THIACYCLOPENTANECARBOXYLATE WHICH COMPRISES,REACTING SAID LOWER ALKYL 2-THIOPHENECARBOXYLATE WITH HYDROGEN IN THEPRESENCE OF A COBALT CARBONYL CATALYST AT A TEMPERATURE OF ABOUT 110*C.TO ABOUT 220*C. UNDER A PRESSURE OF AT LEAST ABOUT 500 P.S.I., WHILEMAINTAINING A CONCENTRATION OF CARBON MONOXIDE IN THE REACTION ZONE ATLEAST SUFFICIENT TO PREVENT DECOMPOSITION OF THE COBALT CATALYST.